In a large production environment, hundreds of machines or stations may make up a production line. The buffer between stations is a storage area such as a conveyor, transport (i.e. forklift, cart, truck, etc.), silo (either manual or automatic), or simply a place within the flow of product. Every station is configured to generate event codes and transmit event data for analysis. Event codes are generated for many different reasons, for example, when a machine is broken, is starved (no parts are available to process), is blocked (unable to unload a completed part), the machine or station takes to long to complete the manufacturing process and the machine or station requires a tool change.
Production efficiency, throughput, is high when a minimum number of parts are on the line or in a specific buffer such as is required only to maintain continuous flow of products through the manufacturing line at the intended speed. If a station breaks down or becomes inoperable, upstream and downstream stations may be quickly affected. Stations upstream may be blocked if buffers they feed are full. Stations downstream may become starved if buffers they draw from are empty.
A breakdown can occur for a number of different reasons. There may be thousands of possible fault codes associated with a machine. When a breakdown occurs, information about the breakdown is stored in the Production Monitoring and Control (PM&C) system. Commonly this includes a reason for the breakdown (called an event code or a fault code) and the duration. The duration might be further subdivided into a response time, a diagnostic time (time to diagnose the problem), an order time (time to order or acquire the replacement part), and the actual time to repair the machine. Other types of subdivisions for times might be appropriate as well.
The term “breakdown” as used herein may include any number of other conditions of a machine. It truly may be a non-operating machine, or inoperable or become offline. However, it may also be non-functional for many different reasons unrelated to the machine itself. In certain situations, stock may become unavailable. For example, an assembly line that receives components to be mounted on the product (crankshafts and pistons for an engine assembly line) from feeder lines may become slowed if components arrive at a machine late. A shortage of these parts also interrupts production, but this situation should not normally occur on a manufacturing line. Also utilities, such as water, electricity, or compress air may become unavailable.
The categories of reasons for a machine not producing parts may vary from line to line and for different types of lines. A manufacturing line has frequent tool changes that cause the interruption of production but which do not normal occur on an assembly line. Other words, including “down” or “downed” may be jargon for any type of inoperability. As used herein, any of the terms or others discussed above are interchangeable. The flow or progress of a line, its throughput, is impacted whether a machine is inoperable, ceases to function or is waiting for parts.
A production facility may have a communications network connecting the stations to a centralized unit. Sensor for determining a station's status and sensors for determining the flow or progress of product making its way through a line may be connected to the centralized unit by wires or they may be wireless. Machine controller may also be connected to the centralized unit. An error or fault code may be generated by sensors or controllers and sent to the centralized unit when a station breaks down or stops operating for any reason. The fault code identifies the reason for the stoppage so that when the fault code and time stamp are available, the time and reason of the breakdown may be determined. A supervisor may be alerted by the centralized unit when a station has stopped operating. Maintenance personnel may be alerted to attend to the downed station. If adjacent buffers become full or empty, other stations may also stop operating. Once a station that has had a breakdown becomes again operational, the line will thereafter start up again. The starved stations will begin receiving product and the blocked stations will be able to process product. However, some stations may be slow to recover after such a problem or fault occurrence.
When all the breakdown data are available, the average duration or MTTR (mean time to repair) and the MCBF (mean count between faults) or MTBF (mean time between faults) and other useful measures can be calculated. Sometimes a curve fit to an analytical function is made. Other times the analytical function is assumed to be a negative exponential.